In the related art, in a projection image display apparatus (projector), a polarization conversion device has been used to enhance light usage efficiency. In this polarization conversion device, a ½ wavelength plate is used to change a polarization direction of light.
With regard to the ½ wavelength plate having such a use, it is necessary to perform superior polarization conversion in the entire wavelengths in a visible region, and to this end, a broadband ½ wavelength plate is used.
Generally, a film made of polycarbonate or the like is used as material of the ½ wavelength plate, but this material is poor in heat resistance or weather resistance and easily deteriorates. Thus, for example, JP-A-2003-302523 discloses a technique which forms a ½ wavelength plate by superposing two sheets of quartz plates.
Further, Japanese Patent No. 4277514 discloses a technique which can form a broadband ½ wavelength plate by superposing two sheets of quartz plates so that their optical axes intersect each other at a predetermined angle.
By forming the ½ wavelength plate with the quartz plates, heat resistance or weather resistance of the wavelength plate is enhanced. However, this causes two problems.
The first problem is that when a polarization conversion device is assembled using the wavelength plate formed by the quartz plates, a problem occurs in heat resistance or weather resistance of the polarization conversion device.
In general, the polarization conversion device has a configuration in which the ½ wavelength plate is fixed onto a plurality of glass prisms which are arranged to form polarization beam splitters. If the ½ wavelength plate is attached onto the glass prisms using an adhesive, the ½ wavelength plate may be separated due to deterioration of the adhesive caused by the heat of the light which passes through the polarization conversion device, or due to the difference between the thermal expansion coefficients of glass and quartz.
In order to solve this problem, JP-A-2009-288262 discloses a technique in which an adhesive is disposed in a peripheral section of a polarization panel formed by glass prisms or the like, that is, in an area which does not transmit light and only the end parts of a ½ wavelength plate are adhesively fixed.
In this technique, since the adhesive is not present in an area which transmits light, deterioration such as separation of the fixed locations does not occur.
Further, in a polarization conversion device disclosed in Japanese Patent No. 4329852, the end parts of a phase difference plate (wavelength plate) are attached to plate glass by a double-sided tape. In this technique, the double-sided tape is not similarly present in an area which does not transmit light. In particular, in Japanese Patent No. 4329852, a minute clearance is formed between the plate glass and the phase difference plate due to the attachment of the double-sided tape, thereby making it possible to suppress occurrence of interference fringes.
The second problem is that it is difficult to provide a large-sized wavelength plate.
In order to manufacture a wavelength plate, it is necessary to prepare a quartz plate suitable for the size of the wavelength plate. Since the quartz plate is formed by cutting a crystal ingot of quartz, a large crystal ingot should be prepared to obtain a large-sized quartz plate. It is difficult to prepare the large crystal ingot, and the cost becomes high even though the large crystal ingot can be prepared, which makes it difficult to provide a large-sized wavelength plate formed of the quartz plate. In this regard, in a polarization conversion device disclosed in JP-A-2007-58018, a plurality of divided quartz plates are positioned and attached to a transparent member to have a function of a single phase difference plate (wavelength plate), thereby providing a large-sized wavelength plate in a pseudo manner.
On the other hand, JP-A-2010-8827 discloses a technique in which the number of divided quartz plates is two. A single phase difference plate (wavelength plate) is formed by the two divided quartz plates.
A phase difference plate supporting member is disposed in a joint position of the two divided quartz plates, the quartz plates are pressed by the phase difference plate supporting member. Thus, the two divided quartz plates are fixed to a polarization splitting device (plate glass) as the single phase difference plate.
Further, both end parts of the phase difference plate formed by the two divided quartz plates are disposed in a peripheral section of the polarization splitting device, that is, in an area thereof which does not transmit light and are fixed to a frame section provided in the periphery of the polarization splitting device by a thermal caulking.
In the technique disclosed in JP-A-2007-58018, each of the plurality of divided quartz plates should be attached to the transparent member by an adhesive. A quartz plate disposed in a center area of the transparent member, in an area which transmits light, is also fixed to the transparent member by the adhesive. Thus, in a similar way to the above-described first problem, a problem may occur that the quartz plate disposed in such an area is easily separated due to deterioration of the adhesive caused by the heat of the light, or due to the difference between the thermal expansion coefficients of glass and quartz.
On the other hand, in the technique disclosed in JP-A-2010-8827, since the single phase difference plate is formed by the two divided quartz plates, a large-sized phase difference plate can be achieved. Further, since both end parts of the phase difference plate are fixed by the caulking, the problem of adhesive deterioration does not occur. Thus, the technique disclosed in JP-A-2010-8827 can solve the above-described two problems.